Knowing whether an antique or vintage lightbulb works or not is a crucial part in determining its value. A collectible lightbulb in non-working condition is worth about 90-95% less than an identical bulb that works. Listing a collectible lightbulb on eBay in "as-is" condition or in untested condition is pretty much synonymous with not wanting to sell the bulb.
Please DO NOT test antique lightbulbs in any lamp socket!!! The voltage used today is higher than what these bulbs were made for (120v versus usually 110-115v). You don't know how many times I've come across an auction description that says: "I tried it in a lamp socket, but it popped, and now it doesn't work." If the bulb "pops", in most cases it will lose ALL of its value (except in the case of extremely rare bulbs; But even these will lose most of their value). Worse, there are some collectible bulbs out there that were made for very low voltage battery-powered systems used in rural areas, and these will be immediately destroyed if they are simply screwed onto any socket. Fortunately, there is a much better, 100% safe way to test old lightbulbs:
First of all, you will need to get hold of a multimeter (also known as an ohmmeter). Most people who like to fix things around the house themselves (aka "the do-it-yourselfers") own one. Also anyone who works daily with electricity has to have one of these. If you aren't confident that you can do this correctly, ask a friend who is into electronics or fixing appliances to help you.
There are digital and analog multimeters. Multimeters are not very expensive; Figure $10 for a simple analog multimeter. Even some digital multimeters are available for the same price.
To test an incandescent lightbulb for continuity:
1.- Connect the probes to the multimeter if necessary.
2.- Rotate the multimeter's switch to where the OHM symbol (Ω) is located (this should turn the multimeter on).
3.- Notice that when you move the probe tips to touch each other, you can see that the multimeter's display changes. When not in contact, the display should show all zeros.
4.- Place one probe on the threaded part of the base of the bulb and the other probe on the round metal plate at the bottom. Do not touch the metal tips of the probes with your fingers while you do this.
5.- See if you get any numbers in the multimeter display.
If the base is a little rusty, try scrubbing it very gently with a dishwashing fiber, and then place the multimeter's probes on the scrubbed metal part.
... And you're done! What you actually did was test the bulb to see if electricity still passes through it. If the filament is broken, chances are it no longer can do this and there is no continuity. If you got nothing but zeros on the multimeter's display, then your bulb probably doesn't work.
Note: Some old lightbulbs don't have threaded bases. In these cases you will need to find out which parts of the base are used for conducting electricity.